The release and synthesis of human growth hormone (GH, somatotropin) are under constant control of two mutually antagonistic hypothalamic hormones—growth hormone release inhibiting hormone (GIH, somatostatin) and growth hormone-releasing hormone. A therapy based on administering human growth hormone-releasing hormone (hGH-RH) may be implemented for majority of patients with GH insufficiency. It has been determined that the shortest fragment, still retaining the full biological activity of endogenous hGH-RH which comprises 44 amino acid residues, is its N-terminal analog hGH-RH(1-29)—NH2. The hGH-RH(1-29)—NH2 peptide can be prepared by synthesis; synthetic GH-RH(1-29)—NH2 under the name sermorelin acetate is approved for use in the treatment of short stature in children and is also under investigation for the treatment of neurosecretion disorders, as an adjuvant for gonadotropin-induced ovulation in infertile women, and for the treatment of AIDS-related catabolic disorders.
It has been disclosed, however, that hGH-RH(1-29)—NH2 is relatively non-resistant to enzymatic degradation. The major metabolites observed are characteristic of bond cleavages between Arg11-Lys12 and Lys12-Val13, caused by trypsin-like enzymes (L. A. Frohman, T. R. Downs, E. P. Heimer, A. M. Felix J. Clin. Invest. 1989, 83, 1533-1540). It has been determined recently that upon degradation with trypsin of an analog of hGH-RH(1-29)-NH2 the hydrolysis of peptide bonds takes place at the carboxylic group site of all basic amino acid residues, including the C-terminal amide bond, while, for an analog differing in that the Orn residues are present instead of Lys, only the bonds neighboring with Arg residues are hydrolyzed (E. Witkowska, A. Orlowska, B. Sagan, M. Smoluch, J. Izdebski J. Peptide Sci. 2000, 6 (Suppl.), 189; E. Witkowska, A. Orlowska, B. Sagan, M. Smoluch, J. Izdebski J. Peptide Sci. 2001, 7, 166-172). This discovery remains in agreement with the exceptionally high in vivo activity of the analogs containing Orn in positions 12 and 21 (J. Izdebski, J. Pinski, J. E. Horwath, G. Halmos, K. Groot, A. V. Schally, Proc. Natl. Acad. Sci. USA, 1995, 92, 4872-4876).
Attempts have been made to overcome the problem of hGH-RH(1-29)-NH2 instability, inter alia by replacing Arg at position 29 in the amino acid sequence with Agm (4-guanidylbutylamine) (Bajusz et al., in Peptides 1982, Blaha and Melon, Eds.; W. De Gruyter, Berlin-New York, pp. 643-647), or replacing Tyr at position 1 with Dat (desaminotyrosine) and Lys at position 12 with D-Lys, Arg or Orn (International Patent Application Nos. WO 94/11396 and WO 94/11397).
The results of these attempts, however, are unsatisfactory and the need still exists for analogs combining enhanced ability to release growth hormone with increased resistance to enzymatic degradation, which would allow diminished doses of the drug and/or a less frequent administration.